1. Field of the Invention
The present invention relates to pneumatic tires having portions which are free of reinforcing fabric and more particularly to laminated pneumatic tires containing hard and soft layers arranged to provide a high resistance to flex cracking.
2. Description of the Prior Art
It has long been recognized in the pneumatic tire industry that fabric reinforced tires have many disadvantages and that it might be desirable to build a fabric-free tire which could be manufactured by less expensive procedures. Not only is the conventional method of tire fabrication slow and expensive, requiring separate handling and processing of one or more kinds of rubber and/or cord fabric for each tire, calendering and handling of individual plies, and final manual assembly of all of the components, but also, the conventional tire construction is not readily adapted to any kind of automation which would reduce the labor costs and improve the tire quality and uniformity. An additional cause of excessive manufacturing expense associated with conventional tire construction is the large amount of factory space taken up, first, by the many large curing presses required to cure green tires, one at a time, for anywhere from about 12 minutes to more than 20 minutes, and second, by the rubber processing and mixing equipment such as mills, Banburys, extruders, calenders, and tire building machines.
Other problems in conventional fabric-reinforced tires are the difficulty of uniform placement of the many components of the tire and the difficulty of obtaining uniform expansion from the cylindrical shape on a flat drum to the final toroidal shape. The laminated tire of the present invention eliminates some of these problems.
In its search for a tire configuration and construction method which does not require the lamination of cords and rubber compound and the subsequent manual buildup of ply layers, the tire industry has attempted to make at least the carcass portion of the tire by molding or casting a liquid polymer, such as a polyurethane, to the desired shape. The tire illustrated in U.S. Pat. No. 2,902,072 comprises a tread, inner and outer sidewalls, and a cellular polyurethane filling the space between the inner and outer sidewalls. U.S. Pat. No. 3,208,500 discloses the fabrication of the carcass portion of a pneumatic tire by molding a polyurethane or polyamide into a single, generally homogeneous layer. U.S. Pat. No. 3,274,322 discloses a method of flow-forming a material such as a polyurethane, using a doctor blade, to make a strip of material which can subsequently be formed into a tire carcass. U.S. Pat. No. 3,386,485 illustrates a method of making a "fabricless" pneumatic tire from any one of a number of suggested polymers and copolymers by separately molding the annular halves of a tire and subsequently molding the two halves together in a different press.
U.S. Pat. No. 3,396,773 illustrates the application of centrifugal casting techniques for making a tire which is described as being solid rather than pneumatic. The general concept of centrifugal casting has been applied to pneumatic tires as illustrated in U.S. Pat. No. 3,555,141 which discloses a method in which a heated mold, having its internal surface conforming to the external shape of a tire, is rotated while a liquid polyurethane is introduced therein. The rotation of the mold causes the polyurethane to flow and cover the entire inner surface of the mold and a stationary template, having the same shape as the inside of the tire, serves to spread the material in the same manner as a doctor blade. In U.S. Pat. No. 3,701,374 there is a disclosure of a fabric-free tire carcass made of a polyurethane elastomer, another elastomer, and methods of making the tire to produce the necessary physical properties.
To the extent pertinent, the above identified U.S. patents are herein incorporated by reference as background information and for supplementing the detailed description appearing below.
While many of the problems associated with conventional tire construction can be reduced or eliminated by fabrication of tires in accordance with the prior art patents discussed above, the cast tire configuration such as the type proposed in these patents has a number of serious problems, and use of such special tires has not been widespread. The major problem in the cast tire, in terms of tire life, is what is known as flex fatigue failure. Under the weight of the vehicle the portion of a tire in contact with the ground is deformed with the result that the rubber is continually flexed during operation of the tire. In conventional fabric reinforced tires, the fabric may for example withstand at least 85 percent of the inflation pressure and the rubber less than 15 percent. If the fabric is to be eliminated without changing the general shape of the tire, then it would be necessary to employ a rubber of higher-modulus to enable the tire to maintain its shape. However, the harder rubbers have a relatively short life due to flex fatigue failure. For example, whenever a small crack or defect occurs due to a microscopic flaw or a cut from a sharp object, there is a tendency for a crack to develop and grow during continual flexing until the tire fails or becomes dangerous. When using fabric reinforcement it is possible to employ softer rubbers which do not have the serious flex fatigue problem. However, when making a fabric-free tire using stiffer polymers, which are required to obtain the necessary strength, the flex fatigue problem is extremely serious. Experimental data with respect to fabric-free tires seems to indicate that sidewall flex cracking is the principle mode of failure.